Ferrite tuned coaxial cavity apparatus



Nov. 15, 1966 J. K. BUTLER FERRITE TUNED COAXIAL CAVITY APPARATUS Filed Nov. 23, 1962 HU -W67 United States Patent 3,286,207 FERRITE TUNED COAXIAL CAVITY APPARATUS Jerome K. Butler, Lawrence, Kans., assignor to Wilcox Electric Company, Inc., Kansas City, Mo., a corporation of Kansas Filed Nov. 23, 1962, Ser. No. 239,639 13 Claims. (Cl. 333-83) This invention relates generally toelectronically tunable cavity resonators and, more specifically, to a magnetically tunable coaxial cavity loaded with ferrite and dielectric materials.

Various schemes have been heretofore employed to provide a resonant circuit operable at frequencies higher than the frequency range associated with the usual capacitor-inductor configuration. Cavities of various types have been utilized including mechanical and electronically tunable types. In the design of cavities of the latter type, various approaches have proven satisfactory for specific applications.

However, in the field of electronically tuned high frequency resonators for transmitting applications, the devices heretofore employed are not entirely satisfactory where space is a primary consideration. Suchis the case when the problem of providing a tunable circuit for the final amplifier of an air-borne transmitter is studied, as well as other applications requiring a comp-act high frequency transmiter assembly capable of generating a high power output. As the physical size of resonant circuit components is reduced, the limitations on power handling capability become acute.

It is, therefore, an object of this invention to provide an electronically tunable high frequency resonator capable of handling the power requirements associated with transmitter applications.

It is another object of this invention to provide an electronically tunable coaxial cavity capable of handling the aforesaid power requirements.

It is still another object of this invention to provide an electronically tunable coaxial cavity loaded with ferrite and dielectrics such that the propagation constant thereof may be controlled by a magnetic field applied to the ferrite.

It is still another object of this invention to provide a coaxial cavity partially loaded with ferrite and dielectric materials that may be tuned over a wide frequency spectrum by varying an applied magnetic field, said cavity invention with parts broken away to show the interior construction thereof; and

FIG. 2 is a view taken along line 2--2 of FIG. 1.

Referring to the figures, it may be seen that the invention comprises a base 10 having a pair of upright supports 12 and 14 secured thereto by bolts 16 and 18, respectively. The view of the interior of the apparatus in FIG. 1 reveals a bore 20 in support 14 which receives an insert 22. The insert comprises a circular disc 24 integral with a stem 26. The circular periphery of disc 24 engages the internal periphery of one end of a cylindrical, tubular conductor 28. A cylindrical bore longitudinally through the insert and axially of the disc 24, receives one end of a solid, cylindrical rod 29, thereby maintaining conductor 28 and rod 29 in coaxial alignment. A setscrew 30 engages stem 26 and secures the insert to support 14.

3,286,207 Patented Nov. 15, 1966 "ice The opposite ends of conductor 28 and rod 29 are similarly secured to support 12 by an insert 31 identical to insert 22. The assembly thus prevents a coaxial cavity configuration with rod 29 forming the inner conductor and conductor 28 forming the outer conductor of a coaxial line.

A slab of dielectric material 32 is disposed between conductors 28 and 29 and extends the longitudinal length of conductor 28 to the inner faces of the inserts. In FIG. 2 the transverse cross-section of slab 32 is shown to be generally sector-shaped. The slab surrounds conductor 29 over approximately a 120 sector. A pair of ferrite strips 34 and 36 also extend longitudinally of conductor 28 between the inserts and form radial projections that engage surfaces 38 and 40 of slab 32.

Magnet poles 42 and 44 are secured to base 10' by such means'as bolts 46 and 48, respectively. Poles 42 and 44 contain windings 50 and 52, respectively. A source of direct current 54 is connected with windings 50 and 52 by conductors 56, 58, 60* and 62. A variable resistance 64 is utilized to control the voltage output of source 54. The windings 50 and 52 in cooperation with their respective poles 42 and 44, comprise D.C. electrom-agnets which are radially aligned with the coaxial conductors along the radial projections formed by ferrite strips 34 and 36.

The closed ends of the coaxial line form the configuration into a cavity suitable for use as a resonator at frequencies determined by the physical length of the cavity. Electromagnetic energy may be coupled with the cavity through the use of coaxial connectors 66 and 68 disposed near the ends of conductor 28. Energy may be introduced into the cavity or coupled therefrom by coupling loops such as illustrated at 70. Loop 70 depends from an aperture 72 in conductor 28. The coaxial connector overlies the aperture to connect the loop with a coaxial cable 74. Loop 70 is connected across the inner and outer conductors of coaxial cable 74, the outer conductor thereof being electrically connected to conductor 28. A coaxial cable 76 is connected to connector 66' and similarly coupled with the cavity. Cables 74 and 76 represent leads from the input and the output of the apparatus,

and such input and output may be interchanged without effecting the operation of the device. This coupling configuration is only illustrative as various conventional approaches may be utilized to satisfactorily couple electromagnetic energy to and from the cavity.

The outer conductor 28 is composed of nonmagnetic material such as brass and may have an inner skin 78 of copper. The conductor 29 is composed of a magnetic material such as soft iron and may have an outer skin 80 of copper. The copper skins afford minimum resistance to a high frequency wave travelling along the coaxial line. Base 10, supports 12 and 14, inserts 22 and 31, and poles 42 and 44 are composed of magnetic material.

The lowest resonant frequency of the cavity is determined by the longitudinal length of conductor 28 between the inner faces of the inserts. The cavity is tuned to higher resonant frequencies by controlling the magnitude of the magnetic field produced by the electromagnets. This may be controlled by adjusting resistance 64.

To understand the operation of the present invention it is first necessary to visualize the propagation of an electromagnetic wave along coaxial conductors. If the ferrite strips and the dielectric slab were removed from the present invention, an electromagnetic wave travelling along the conductors would produce an electric field radial of the conductors and a magnetic field that moves in an angular path around the axis of the conductors and through the space therebetween. No component of the magnetic field is longitudinal of the coaxial conductors. In other words, a coaxial line is only linearly polarized with regard to the magnetic field of the electromagnetic wave travelling therealong.

It may be seen in FIG. 2 that a spherical coordinate system is arranged with its origin at the axis of the coaxial conductors. The component of the spherical coordinate system is measuredfrom a base line extending from the origin and labeled x. The r component of the coordinate system is measured from the origin along lines extending radially outwardly therefrom. The z axis of the coordinate system extends from the origin along the axis of the coaxial conductors and thus cannot be seen in the two dimensional representation of FIG. 2, as the z axis extends perpendicularly of the plane of the drawing.

Maxwells equation is as follows:

DE. ar

where H =z magnetic field component E,=='radial electri field vector Thus, it may be seen that a component of the magnetic field along the z axis will be produced at some angle 9 Where there is a change in the magnitude of the radial electric, field vector. The equation states that this z component of the magnetic field is proportional to the rate of change of the radial electric field vector with respect to the angle 9. The magnitude of the radial electric field vector is different for different dielectric constants. In the present invention the dielectric constant is dependent upon the angle 6 because at angles 0 and 0 shown in FIG. 2, the dielectric constant changes from that of the ambient medium between conductors 28 and 29 to the dielectric constant of the slab 32. Therefore, Maxwells equation proves that a magnetic vector longitudinally of the coaxial line will be obtained at the boundary surfaces in the line (surfaces 38 and 40) where there is a change in dielectric constant.

With the ferrite strips and the dielectric slab removed from the coaxial conductors, the resonant frequency of the cavity will be dependent upon its physical length. However, if the propagation constant of the coaxial cavity is varied in some manner, the resonant frequency thereof will change even though the physical length remains constant.

Qne of the many factors which determines the value of the propagation constant its the a of the medium between the conductors. Ferrite materials undergo a change in permeability when this material is subjected to a magnetic field. This change is due, theoretically, to the alignment of the electrons in the ferrite strips with the applied magnetic field. This change is especially pronounced at values of magnetic field intensity close to but less than the intensity required to magnetically saturate the ferrite.

If a ferrite material is placed in a direct current magnetic field, the spin axes of the electrons in the ferrite become aligned with the lines of force of the applied D.C. field. Theoretically, the electrons precess about their aligned axes in an orbital plane normal to the applied field. As the strength of the field is increased the spin of the electrons around their individual axes becomes more rapid and, also, the orbit becomes smaller. In other 'words, the angle of procession drawn from the aligned axis of the electron ,to the orbital path thereof becomes increasingly acute until, theoretically, the orbit at magnetic saturation is zero; The change in the orbital paths of the precessing electrons changes the permeability (a) of the ferrite material. At magnetic saturation the ,u of a ferrite is theoretically infinite. However, it Will not actually be infinite due to losses.

In order for this phenomenon to be utilized to control the propagation constant of the coaxial cavity, the precessing electrons of the ferrite strips must be disposed to interact with the magnetic field of the wave being propagated along the coaxial conductors. It will be seen in FIG. 1

that the base 10, supports 12 and 14, and inserts 22 and 31, being composed of magnetic material, interconnect the ends of magnetic conductor 29with the poles 42 and 44 of the electromagnets. The electromagnets are wound such that the tips 82 and 84 of the electromagnets are of the same magnetic polarity. The magnetic circuit will thus traverse a path from the tips 82 and 84 radially along the ferrite strips 34 and 36 to the inner conductor 29 and hence along the supports 12 and 14 to the base 10 and the electromagnets. A reversal in the polarity of tips 82 and 84 would, of course, reverse the direction of the flow of the magnetic flux. In either event, the effect of the magnetic circuit is to align the axes of the electrons in the ferrite strips in a direction radial of the coaxial conductors.

'Ilhe precessing electrons are caused to interact with the magnetic field of the electromagnetic wave being propagated along the coaxial conductors by the action of the dielectric slab 3-2. As described above, a magnetic component longitudinal of the coaxial conductors will be produced at the boundary surfaces between slab 32 and the ambient medium between the conductors. (This medium may be air, but is should be understood that another medium or material may be used instead of the air space, it only being necessary that this other medium or material have a dielectric constant substantially different than that of slab 32.) Therefore, at surfaces 38 and 40 the magnetic vector will not trace an ellipse rather than a line as such vector comprises components both in the z direction longitudinally of the coaxial conductors and in the 19 direction. This elliptical polarization lies in planes parallel with the z axis, or axis of the coaxial conductors, and normal to surfaces 38 land 40. Since the planes of the precession orbits of the electrons in the ferrite strips are normal to the applied magnetic field, the precession planes and the polarization planes will substantially coplanar at the surfaces 38 and 40 that abut the ferrite strips.

The magnetic field of the propagated wave will now interact with the ferrite strips due to this coplanar relationship. The interaction causes the propagatio constant of the cavity to be varied in accordance with the intensity of the applied D.C. magnetic field from the electromagnet-s. Interaction between the propagating wave and the precessing electrons is especially predominant when circular polarization of the magnetic field of the propagated wave is obtained.

The invention has been illustrated and hereinabove described from the standpoint of conductors that are essentially circular in transverse cross-section and coaxially aligned; hence, the use of the term radial. It is manifest, however, that I do not desire to be limited to such exact cross-sectional configurations in either of the conductors nor to radial dispositions of any of the components under the technical definition of radial applied only to circular shapes. The coaxial alignment is also not critical, although such an alignment simplifies the design of the apparatus to meet the requirements of a particular application and aids in the prediction of its operational characteristics. Furthermore, although the precession theory is used in the above discussion to explain the interaction of the [ferrite 'with the magnetic field of the propagated wave, it should be understood that I do not intend that the teachings of the present invention be limited to any particular operational theory.

The frequency range of the resonator will depend upon the physical length of the cavity and the composition of the particular ferrite strips. When ferrite strips composed of yttrium-gallium iron garnet crystals are utilized, the cavity is especially suited for frequencies of approximately 1,000 megacycles per second. In general, the greater the ferrite is enriched in gallium the lower will be the possible upper limit of the resonant frequency range of the cavity. The specific crystal composition of the yttrium-gallium iron garnet crystal is where the value of X is in the range of approximately 0.14 to 0.20.

In the practice or the invention, the dielectric slab 32 should be a material having a substantially different dielectric constant than the dielectric constant of the medium or the substance filling the remainder of the space between the coaxial conductors. As a result of Maxwells equation, the difference in dielectric constant is one factor which determines the ellipticity of the polarization and, therefore, to what degree circular polarization will be achieved. For use in an ambient medium of air, a material dielectric constant on the order of 15 may be used for the dielectric slab.

It should be understood that the present invention is operable with the magnetic return path along supports 12 and 14 removed and the tips '82 and 84 of the electromagnets maintained at opposite magnetic polarities. When thus operated, the magnetic lines of force traverse radially through the ferrite strips, the conductor 29 serving to magnetically interconnect the radially innermost portions of the [ferrite strips to provide a low reluctance t-herebetween. However, such operation will result in nonreciprocal propagation. This will increase the insert-ion loss of the cavity and lower the tuning range to approximately ten percent on each side out the center frequency.

Furthermore, the invention is operable with only one end of the cavity closed rather than with two closed ends as shown. Again, the efiiciency of the apparatus will be lowered.

Having thus described the invention, what is claimed as new and desired to be secured by Letters Patent is:

1. Resonator apparatus comprising:

a pair of spaced electrical conductors, there being an outer, hollow conductor having a transverse member therein and an inner conductor electrically coupled to the member and disposed in said outer conductor to present a cavity resonator;

an electromagnetic energy input means and an electromagnetic energy output means, each operably coupled with said conductors;

a pair of dielectrics of ditferent dielectric constant in said cavity, each dielectric partially surrounding the inner conductor and each having a pair of surfaces, each surface lying in a plane intersecting said inner conductor and normal to a plane transverse of said inner conductor, each surface of each dielectric being adjacent to a corresponding, opposing surface of the other dielectric to form a pair oi? spaces between the opposing surfaces, whereby to elliptically polarize the magnetic field of an electromagnetic wave traveling along the conductors in the regions of the opposing surfaces;

a strip of material disposed in each space respectively in contact with said opposing surfaces, said material being characterized by atomic structure wherein the electrons of the atomic structure will align their individual spin axes with an applied magnetic field and precess about said axes in :orbital planes normal to the aligned axes; and

means operable to direct magnetic field components along said strips transversely thereof and either exclusively toward said inner conductor or away 'irom the latter, whereby to impart reciprocal propagation characteristics to the cavity and, align said spin axes to produce interaction of the precessing electrons with said elliptically polarized magnetic field to thereby control the propagation constant of the cavity.

2. Apparatus as set forth in claim 1, wherein said material is a ferrite substance.

3. Apparatus as set forth in claim 2, wherein the ferrite comprises yttrium-gallium iron garnet crystals.

4. Apparatus as set forth in claim 1, wherein the outer conductor is generally cylindrical and the inner conductor is disposed in said outer conductor in substantially coaxial alignment therewith.

5. Apparatus as set 'forth in claim 4, wherein the dielectrics are generally transversely sector-shaped, said sector-shapes being approximately degree and 240 degree sectors, respectively.

6. Apparatus as set forth in claim 5, wherein said mateiral is a ferrite substance.

7. Resonator apparatus comprising:

a coaxial cavity resonator including a pair of spaced electrical conductors, there being an outer, hollow, cylindrical conductor of nonmagnetic material and an inner conductor of magnetic material disposed in said outer conductor in coaxial alignment therewith, and a pair of transverse members closing the ends of the outer conductor and electrically interconnecting the inner and outer conductors;

and electromagnetic energy input means and an electromagnetic energy output means, each operably coupled with said conductors;

a pair of ierrite strips disposed longitudinally and radial-1y betwee the inner and outer conductors to divide the annular space between the conductors into two sections;

a generally transversely sector-shaped dielectric disposed in each section respectively in contact with a transverse surface of each strip, the dielectric constant of each dielectric being substantially different than the dielectric constant of the other dielectric; and

magnet means operable to direct D.C. magnetic field components along said rferrite strips generally radially of the coaxial conductors to thereby control the propagation constant of the cavity.

8. Apparatus as set forth in claim 7, wherein said field components are directed in like radial directions, and wherein is included magnetic circuit means op'erably coupling the inner conductor to said magnet means to provide a closed magnetic circuit for said field components.

9. Apparatus as set forth in claim 7, wherein the transverse sector-shapes of the dielectrics are approximately 120 degree and 240 degree sectors respectively.

10. Apparatus as set forth in claim 9, wherein the 240 degree sector dielectric is air and the 120 degree sector dielectric consists of a material having a dielectric constant of approximately 15.

11. Apparatus as set forth in claim 7, wherein the territe in the strips comprises yttriumaga'llium iron garnet crystals.

12. Apparatus as set forth in claim 11, wherein said crystals comprise 3Y O' 5 [(1.OX)Fe O' -XGa O where the value of X is in the range of approximately 0.14 to 0.20.

13. Apparatus as set forth in claim 1, wherein said inner conductor comprises a magnetic substance, said component-directing means including means coupled with said inner conductor for establishing a closed magnetic circuit through the cavity for said components.

References Cited by the Examiner UNITED STATES PATENTS 3,048,801 8/ 1962 -Fegel et al. 333--24.2 3,072,867 1/ 1963 Heitha-us 3 33-24.2 3,028,425 2/ 1963 Duncan 3 3:324.2 3,225,318 12/1965 Heithaus 33324.2

HERMAN KARL SAALBACH, Primary Examiner.

E. LIEBERMAN, Assistant Examiner. 

1. RESONATOR APPARATUS COMPRISING: A PAIR OF SPACED ELECTRICAL CONDUCTORS, THERE BEING AN OUTER, HOLLOW CONDUCTOR HAVING A TRANSVERSE MEMBER THEREIN AND AN INNER CONDUCTOR ELECTRICALLY COUPLED TO THE MEMBER AND DISPOSED IN SAID OUTER CONDUCTOR TO PRESENT A CAVITY RESONATOR; AN ELECTROMAGNETIC ENERGY INPUT MEANS AND AN ELECTROMAGNETIC ENERGY OUTPUT MEANS, EACH OPERABLY COUPLED WITH SAID CONDUCTORS; A PAIR OF DIELECTRICS OF DIFFERENT DIELECTRIC CONSTANT IN SAID CAVITY, EACH DIELECTRIC PARTIALLY SURROUNDING THE INNER CONDUCTOR AND EACH HAVING A PAIR OF SURFACES, EACH SURFACE LYING IN A PLANE INTERSECTING SAID INNER CONDUCTOR AND NORMAL TO A PLANE TRANSVERSE OF SAID INNER CONDUCTOR, EACH SURFACE OF EACH DIELECTRIC BEING ADJACENT TO A CORRESPONDING, OPPOSING SURFACE OF THE OTHER DIELECTRIC TO FORM A PAIR OF SPACES BETWEEN THE OPPOSING SURFACES, WHEREBY TO ELLIPTICALLY POLARIZE THE MAGNETIC FIELD OF AN ELECTROMAGNETIC WAVE TRAVELING ALONG THE CONDUCTORS IN THE REGIONS OF THE OPPOSING SURFACES; 